Cellulose esters are valuable polymers that are useful in many plastic, film, coating, and fiber applications. Cellulose esters (CEs) are typically synthesized by the reaction of cellulose with an anhydride or anhydrides corresponding to the desired ester group or groups, using the corresponding carboxylic acid as diluent and product solvent. Some of these ester groups can afterward be hydrolyzed to obtain a partially-esterified product. These partially substituted cellulose esters have great commercial value, and find use in coatings, where their greater solubility and compatibility with co-resins (in comparison with triesters) and hydroxyl group content (to facilitate crosslinking) are prized.
Paint manufacturers, especially automotive original equipment manufacturers (OEM) strive to gain competitive advantage over each other by focusing on improving the overall appearance. Exterior color and overall appearance can play a major role in influencing the buying process of OEM end users. On the other hand, environmental legislation across the globe has resulted in the paint industry attempting to move towards greener and more ‘eco-friendly’ products with similar performance attributes as conventional coating formulations. Increases in the total solids content or a replacement of certain organic solvents by water are two alternatives available to the coating suppliers to limit the amount of volatile organic compounds (VOC) in the paint formulation. OEM producers that use high-solids basecoat technology want to compete in terms of color brightness and overall aesthetics with those using waterborne basecoat technologies.
Waterborne basecoats are typically applied at 20-25% total solids, which is much lower than the non-volatile content of solvent borne high-solids (HS) coatings. Therefore, HS solvent borne basecoat technology is challenged in terms of the film shrinkage that it can bring when compared to waterborne systems. Lower film shrinkage in the case of HS basecoats leads to poorer alignment of metal flakes and reduced metallic travel. An unmet need exists for a solution that can enable OEM producers using solvent borne technology to develop basecoats with brighter colors and improved overall appearance. This invention provides for the use of low molecular weight cellulose mixed esters, which provides excellent appearance properties in high solids OEM basecoat applications. In one embodiment of the invention, the low molecular weight cellulose ester has a 29% butyryl (Bu) and a 1.5% OH content. The relatively high Tg of about 110° C. coupled with the low butyryl content enables the low molecular weight cellulose ester to provide performance benefits very similar to commercial CABs without causing a detrimental effect on the VOC levels.
Any substrate material can be coated with the basecoat composition according to the present invention. These include surfaces, such as, but not limited to, glass, ceramic, paper, wood and plastic. The basecoat composition of the present invention is particularly adapted for metal substrates and specifically for use in a basecoat application. The basecoat composition may be applied using conventional spray equipment such as air-atomized guns or electrostatic bell applicators.